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Investment Securities Available for Sale - Narrative (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Schedule of Investments [Line Items] |  |  |  |  |
Proceeds from sales of investment securities | $ 16,800,000 | $ 15,400,000 | $ 26,900,000 | $ 52,700,000 |
Realized net losses on sales of investment securities available for sale | 8,800,000 | 600,000 | $ 12,300,000 | 2,300,000 |
Contractual maturities (up to) |  |  | 30 years |  |
Weighted average life of available for sale securities portfolio |  |  | 5 years 339 days |  | 7 years 47 days
Gross unrealized losses | (44,647,000) |  | $ (44,647,000) |  | $ (15,215,000)
Other-than-temporary impairment recorded in earnings | 0 | $ 0 | 0 | $ 0 |
Agency RMBS |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |
Gross unrealized losses | $ (44,637,000) |  | $ (44,637,000) |  | $ (15,197,000)

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